US 20090325014 A1
An electrolysis cell is controlled for operation under varying electrical power supply conditions. A flow of feed stock to the cell includes an electrolysis reactant at a controlled concentration. A varying amount of electrical power is supplied to the cell to produce an electrolysis reaction that generates a first reaction product at a first side of the cell and a second reaction product at a second side of the cell. The reactant concentration is adjusted as the electrical power varies to substantially maintain the cell at its thermal neutral voltage during cell operation. The cell may be used in an electrolysis system powered by a renewable energy source with varying power output (e.g., wind, solar, etc.).
1. A method for operating an electrolysis cell under varying electrical power supply conditions, comprising:
providing a flow of feed stock to a first side of said cell, said feed stock comprising an electrolysis reactant at a controlled concentration;
supplying varying electrical power to said cell to produce an electrolysis reaction that generates a first reaction product at a first side of said cell and a second reaction product at a second side of said cell; and
adjusting said reactant concentration as said electrical power varies to substantially maintain said cell at its thermal neutral voltage during cell operation.
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8. An electrolysis system, comprising:
an electrolysis cell;
an feed input providing a flow of feed stock to said cell, said feed stock comprising an electrolysis reactant at a controlled concentration;
a power supply supplying varying electrical power to said cell to produce an electrolysis reaction that generates a first reaction product at a first side of said cell and a second reaction product at a second side of said cell; and
a controller adjusting said reactant concentration as said electrical power varies to substantially maintain said cell at its thermal neutral voltage during cell operation.
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19. A method for operating a solid oxide electrolysis cell under varying electrical power supply conditions, the cell being constructed for electrolysis of water and having an electrolyte, a hydrogen electrode on a first side of the electrolyte and an oxygen electrode on a second side of the electrolyte, said method comprising:
providing a flow of feed stock to said hydrogen side of said cell, said feed stock comprising steam at a controlled humidity level;
supplying varying electrical power to said cell to produce an electrolysis reaction that generates hydrogen at said hydrogen side of said cell and oxygen at said oxygen side of said cell;
adjusting said humidity level as said electrical power varies to substantially maintain said cell at its thermal neutral voltage during cell operation;
said humidity level being increased as said electrical power increases and said humidity level being decreased as said electrical power decreases; and
said humidity being varied without changing a flow rate of said feed stock.
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This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/052,618, filed on May 12, 2008 (the '618 application). The contents of the '618 application are hereby incorporated by this reference in their entirety as if fully set forth herein.
This invention was made with Government support under contract W31 P4Q-07-C-0321 awarded by US Army Aviation & Missile Command. The Government has certain rights in the invention.
1. Field of the Invention
The present invention relates to electrolysis cells and their operation. More particularly, the invention pertains to the operation of electrolysis cells under conditions of varying power input.
2. Description of the Prior Art
By way of background, an electrolysis cell has electrochemical properties that allow for the conversion of electrical energy into chemical energy. For example, water, in the form of steam, can be converted into hydrogen and oxygen when electrical energy is applied to the cell. As the electrical energy passes through the cell, the electrical resistance of the materials that make up the cell cause some of the electrical energy to be converted into heat (thermal energy). This thermal energy can be used in the electrolysis reaction. As the supplied electrical energy increases, a point is reached where the thermal energy generated within the cell and the supplied electrical energy equals the energy required to complete the reaction. This is called the thermal neutral voltage (Vtn).
For typical applications where the amount of available electrical power (i.e., energy rate) is constant, an electrolysis cell can be designed to run at Vtn. However, for applications where the electrical power is changing over time, as is the case for some renewable energy sources (e.g., wind turbines, solar panels, etc.), changes to cell operating conditions will occur. A specific electrolysis cell will have a defined voltage-current (electrical power) curve. Therefore, if no other operating parameters change, as the input electrical power changes, both the voltage and the current must change. The result is that the electrolysis cell may not always be operating at Vtn. If the electrical power increases, the voltage will exceed Vtn, causing excess heat to be generated in the electrolysis cell that must be removed. If the electrical power decreases, the voltage will be less than Vtn, and heat must be added to complete the reaction. Although there are methods to add additional heat or to remove excess heat, all such methods result in a thermal gradient in the cell. Thermal gradients result in stresses and are a cause of failure.
An electrolysis cell for the production of chemical energy from electrical is controlled for operation under varying electrical power supply conditions. A flow of feed stock to the cell includes an electrolysis reactant at a controlled concentration. A varying amount of electrical power is supplied to the cell to produce an electrolysis reaction that generates a first reaction product at a first side of the cell and a second reaction product at a second side of the cell. The reactant concentration is adjusted as the electrical power varies to substantially maintain the cell at its thermal neutral voltage during cell operation.
According to a disclosed embodiment, the electrolysis cell may be constructed a solid oxide electrolysis cell and water may be used as the reactant to provide an electrolysis reaction that produces hydrogen and oxygen as the reaction products. According to a further embodiment, an electrolysis cell (or a stack of electrolysis cells) may be used in an electrolysis system powered by a renewable energy source. The electrolysis cell or stack may comprise a fuel cell or stack that can be selectively operated in an electrolysis mode and in a power generating mode. One or both of the reaction products produced during the electrolysis mode can be recovered and recycled. In the power generating mode, recovered reaction product may be used as feed material. In the electrolysis mode, recovered reaction product may be used to condition the feed stock to promote electrolysis.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying Drawings, in which:
Turning now to
The cell 100 includes an electrolyte 10 with a first electrode 20 located on one side of the electrolyte and a second electrode 30 located on the other side. During electrolysis operation, an electrolysis reactant 110 will pass over the first electrode 20. When electrical power is supplied to the cell (i.e. across the first electrode 20 and the second electrode 30), a catalyst in the first electrode 20 separates the reactant 110 into ions 120 of a second reaction product 130 and a mixture 140 of a first reaction product and unreacted reactant. The ions 120 pass though the electrolyte 10 and the second reaction product 130 is formed at a catalyst in the second electrode 30. If the cell 100 is a SOEC designed to electrolyze water, the first electrode 20 may be referred to as the hydrogen electrode, the second electrode 30 may be referred to as the oxygen electrode, and the reactant 110 will be high temperature steam. The catalyst in the hydrogen electrode 20 will separate the steam 110 into oxygen ions 120 and a mixture of hydrogen gas and unreacted steam 140. As the oxygen ions 120 pass though the electrolyte 10, oxygen gas 130 will be formed at the catalyst in the oxygen electrode 30.
For water electrolysis, the materials and construction techniques that may be used to fabricate the electrolyte 10, the hydrogen electrode 20 and the oxygen electrode 30 are well known in the art. As such, the details of such materials and construction techniques are omitted in order not to obfuscate the present discussion. Similarly, it will be appreciated that other materials could be used to construct electrolysis cells designed to promote other electrolysis reactions for producing different reaction products. One example is the electrolysis of carbon dioxide into carbon monoxide and oxygen. Again, the materials and construction techniques that may be used to fabricate such electrolysis cells are well known, and will not be described herein.
A power supply 145 provides the electrical power to the cell 100. Examples of energy sources that may be used as the power supply 145 include, but are not limited to, an electrical grid, an electrical generator and a renewable energy source such as a wind turbine, a solar panel array, etc. As described in more detail below, it is assumed that the electrical power available from the power supply 145 varies over time, such that the power supply delivers a varying amount of power to the cell 100.
The electrical performance characteristic of the cell 100 is characterized by a V-I plot of voltage (V) versus current density (A/cm2). An example of such a plot 150 for a solid oxide electrolysis cell construction is shown in
As described by way of background above, an electrolysis cell such as the cell 100 has a thermal neutral voltage Vtn wherein the thermal energy generated within the cell and the supplied electrical energy equals the energy required to complete the electrolysis reaction. The Vtn for a solid oxide electrolysis cell construction is approximately 1.28 Volts. As also described by way of background above, whenever an electrolysis cell is operating at less than the Vtn, thermal energy is required. This adds additional complexity because heaters are required to maintain operating temperature. When operating at higher than Vtn, excess heat is generated and cooling is needed. Therefore, any time an electrolysis cell is not operating at Vtn, additional energy and/or hardware is required to maintain proper operation. Although there are methods to add additional heat or remove excess heat, all the methods result in a thermal gradient within the cell. A solid oxide electrolysis cell typically operates in the range of 650° C. to 1000° C. depending on the specific material used to construct the device. In this environment, thermal gradients can result in severe stresses that lead to eventual failure, especially in a multi-cell stack.
The foregoing considerations are not particularly problematic when the electrolysis cell is powered from a land-based electrical energy source. In that application, the energy source will provide a relatively constant level of electrical power at a relatively constant voltage. However, the situation is more complicated when the electrical power is provided by variable energy sources such as wind turbines, solar cell arrays, or other renewable energy sources.
In such applications where the electrical power is not constant, it is problematic to use an electrolysis cell designed for a single operating point based on the assumption that a constant power source available. It is instead proposed herein that the operating point of the electrolysis cell should be dynamically changed in response to input power fluctuations in order to substantially maintain the cell at its Vtn during cell operation, thereby minimizing thermal gradients and obviating the need for adding or removing heat. The preferred technique is to adjust the concentration of the electrolysis reactant in the feed stock supplied to the electrolysis cell as the incoming electrical power changes. This concentration will typically be a volume percentage of the reactant relative to total feed stock volume, but could also be calculated as a weight percentage. For example, assuming the cell 100 is a SOEC designed to operate with steam as the electrolysis reactant, adjustment would be made to the moisture content of the feed stock as the power provided by the power supply 145 rises and falls.
According to Table 1, when the concentration of water is 33%, plot line 170 governs and the electrical power required to operate at Vtn is 0.66 Watts, which corresponds to a current density at the Vtn crossing point 182 of −0.51 Amps/cm2. As the steam concentration changes to 50%, the plot line 150 governs, the required power changes to 0.72 Watts and the current density at the Vtn crossing points 184 is −0.56 Amps/cm2. As the electrical power is further increased to 0.81 watts, the steam concentration must be 63% and the plot line 160 controls, such that the current density at the Vtn crossing point 186 is −0.63 Amps/cm2. Additional data points and curve fitting would provide an equation that would model the relationship between electrical power at constant Vtn and H2O (steam) concentration.
As can be seen, by adjusting the feed stock moisture content in response to fluctuations in available electrical power, the current required to operate at the thermal neutral point is changed, thereby maintaining the desired thermal neutral voltage level Vtn. As the input electrical power rises, the steam concentration is correspondingly increased. As the input electrical power falls, the steam concentration is correspondingly decreased. It will be appreciated that the precise relationship between steam concentration and the crossing point of the Vtn line 180 is dependent on the specific construction of the cell 100, but may be readily determined and characterized, as indicated above. Similar relationships for other electrolysis cells using other reactants may likewise be determined.
Turning now to
A control and power management system 290 (which may be computerized) provides a controller that monitors incoming electrical power available to the cell 100 and controls the metering pump 260 to provide the required amount of water from the liquid H2O storage tank 250 to adjust the humidity level (i.e., concentration) of the steam 110 flowing to the cell relative to the total amount of feed stock, which includes the recycled hydrogen and possibly other components (e.g., nitrogen), preferably without changing feed stock flow rate.
One application where the system 200 may be used is in long flight duration aircraft that includes a renewable energy source such as on-board solar panels. For such aircraft, the system 200 can be used to produce hydrogen utilizing electrical power from the solar panels. As H2O gas flows through the cell 100, it is converted to H2 and O−2. The O−2 pass through the membrane and exists the cell as O2. The ability of the system 200 to recover the H2 and the O2 is advantageous. The recovered hydrogen reaction product can be recycled back to the cell 100 and used to condition the feed stock to promote the electrolysis reaction. The recovered oxygen can be used to support long duration high altitude flight.
Further versatility may be obtained when the cell 100 is constructed as a fuel cell capable of reverse operation as an electrolysis cell. During periods of time when electrical power is needed (no longer available from the solar panels) the operation of the cell 100 can be reversed to the fuel cell mode. In that case, the hydrogen generated in the electrolysis mode is recovered and recycled as fuel to generate electrical power. The oxygen generated in the electrolysis mode can be similarly recycled to the fuel cell to supplement the ambient oxygen available in or around the aircraft.
Accordingly, technique for operating an electrolysis cell under conditions of varying electrical power, together with an example electrolysis system, have been disclosed. Although various embodiments have been described, it should be apparent that many variations and alternative embodiments could be implemented in accordance with the invention. It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.